1. Field of the Invention
The present invention relates to a screen printing method and a screen printing apparatus primarily for printing a print-use paste, such as solder paste or pastes for thick-film circuit formation, onto a board, such as a printed circuit board, for electronic circuit formation.
2. Description of the Related Art
In recent years, screen printing apparatuses have been used for the solder paste printing process in circuit assembly processes of electronic components, or the like. As the board for electronic circuit formation is advanced toward a further fine structure with the miniaturization of electronic equipment, there has been a demand for higher precision of printing with solder paste or the like responsively.
FIG. 16 shows a conventional screen printing apparatus.
A print-object article (article to be printed) 1 is positioned and fixed to a positioning stage 2 that can be ascended and descended by a positioning stage ascent/descent driving means 3. For printing process, the positioning stage 2 is lifted by the positioning stage ascent/descent driving means 3 to such an extent that the top surface of the print-object article 1 comes in near contact with the bottom surface of a stencil 4.
A left-squeegee ascent/descent driving means 6a and a right-squeegee ascent/descent driving means 6b, which are commonly implemented by double-rod air cylinders, have squeegees 5a, 5b attached to their ends. Lower-limit positions of the squeegees 5a, 5b, (i.e., the push-in strokes to the stencil 4) are set by positional adjustment of stoppers 7a, 7b. A drive source for a horizontal reciprocation driving means 8 is commonly an AC servo motor. In the state that the left squeegee 5a and the right squeegee 5b have descended into contact with the top surface of the stencil 4, the horizontal reciprocation driving means 8 moves the left squeegee 5a and the right squeegee 5b horizontally (in the X-direction), so that a print paste 9, such as solder paste, is moved on the top surface of the stencil 4 by the left squeegee 5a and the right squeegee 5b.
The printing process is carried out as shown in FIG. 17.
At the first step, the print-object article 1 is positioned and fixed to the positioning stage 2. At the second step, the print-object article 1 is lifted to a proximity of the bottom surface of the stencil 4. The left squeegee 5a is lowered at the third step, and the left squeegee 5a is moved rightward in the X-direction at the fourth step, by which printing is executed.
Thereafter, the print-object article 1 is separated away from the stencil 4 at a low speed (20 m/sec) at the fifth step, the left squeegee 5a is lifted at the sixth step, and the print-object article 1 is removed at the seventh step.
Next, a printing operation on the right squeegee 5b side is also executed in the same way as above, and the operation is then alternately repeated.
The drive source for the positioning stage ascent/descent driving means 3 is commonly an air cylinder, pulse motor, AC servo motor, or the like. Among these, the AC servo motor is particularly suitable, because low-speed descent of the driving means 3 allows an easy accomplishment of high-precision printing results with less spread and blurs (i.e., less making indistinct and hazy in outline). The left squeegee 5a and the right squeegee 5b are given mainly by elastic material, commonly urethane rubber (hardness: Hs 60-90.degree.).
In this way, successful prints free from spread and blurs can be obtained continuously. However, there are issues as shown in FIGS. 18A to 18C.
FIG. 18A shows a completion state of the fourth step, FIG. 18B shows an early-stage state of the fifth step, and FIG. 18C shows a last-stage state of the fifth step. In the completion state of the fourth step as shown in FIG. 18A, a print pattern 11a has been formed with relatively good precision. In this state, the left squeegee 5a is curved only by a portion corresponding to the push-in stroke into the stencil 4.
In FIG. 18B, the left squeegee 5a pushes down the stencil 4 to an extent of the push-in stroke, so that the stencil 4 is tilted, causing the print pattern to gradually collapse as shown by a print pattern 11b. As a result, as shown in FIG. 18C, a horn 10 with the print paste 9 lifted is formed at a corner portion of a print pattern 11c.
The horn 10 of the print paste 9 would gradually bow, and drop onto the print-object article 1, causing printing faults as an issue.
As an example, in electronic component assembling processes typified by solder paste printing, the solder paste bowed and dropped after the subsequent-process soldering reflow would cause soldering faults such as solder balls and solder bridges.
In recent years, in the fields of screen printing methods and apparatuses therefor, there have increasingly been cases where screen printing is executed on boards on which an area corresponding to a large opening area of a screen metal mask (stencil) and an area corresponding to a minute opening area thereof are mixedly present in circuits. The term "minute opening area" refers to an area where the value of each opening size of openings of the mask along the squeegee's moving direction is smaller than a specified threshold. The term "large opening area" refers to an area where the value of each opening size of openings of the mask along the squeegee's moving direction is not smaller than a specified threshold.
Now a case where a board in which an area corresponding to a large opening area of the mask and an area corresponding to a minute opening area thereof are mixedly present in circuits is screen-printed by a conventional screen printing method and apparatus therefor is described with reference to FIGS. 19 to 22.
Referring to FIG. 19, which is a perspective view of the screen printing apparatus, reference numeral 101 denotes a screen metal mask; 102 denotes a board; 803 denotes a print head; 104 denotes a print-head-use AC servo motor for driving the print head 803; 105 denotes a print-head-use ball screw for transferring the driving force of the print-head-use AC servo motor 104; 106 denotes a print-head-use AC servo driver for driving the print-head-use AC servo motor 104; 107 denotes a visual recognition camera for recognizing recognition marks of the screen metal mask 101 and the board 102; 108 denotes a recognition-camera-use AC servo motor for driving the visual recognition camera 107; 109 denotes a recognition-camera-use ball screw for transferring the driving force of the recognition-camera-use AC servo motor 108; 110 denotes a recognition-camera-use AC servo driver for driving the recognition-camera-use AC servo motor 108; 811 denotes a controller for issuing commands to the individual servo motor drivers; 112 denotes a control panel for entering data into the controller 811; 113 denotes a stage for restricting the board 102; 114 denotes a stage-use AC servo motor for driving the stage 113; 115 denotes a stage-use ball screw for transferring the driving force of the stage-use AC servo motor 114; 116 denotes a stage-use AC servo driver for driving the stage-use AC servo motor 114; 117 denotes a loader for carrying in the unprinted board 102; 118 denotes an unloader for carrying out the printed board 102; and 119 denotes the main unit of the screen printing apparatus.
Referring to FIG. 20, which is a plan view of the screen metal mask 101, reference numeral 120 denotes a large opening area where solder paste in large openings of the mask 101 are printed on the board 102 (e.g. an area corresponding to a chip component area of the board 102) and 121 denotes a minute opening area, where solder paste in minute openings of the mask 101 are printed on the board 102 (e.g. an area corresponding to a narrow-pitch QFP (Quad Flat Package) area of the board 102).
The operation of the screen printing apparatus which employs a conventional screen printing method is explained with reference to FIGS. 19 to 22.
At Step #901 of FIG. 22, which shows the flow chart of the conventional example, an operator, with the use of the control panel 112, enters descent/ascent positions, i.e. a print start position and a print end position, of a squeegee 123 of the print head 803 in accordance with the screen metal mask 101, and enters a print speed V that allows the circuits of the board 102 to be printed stably and appropriately. In this case, the print speed V that allows a stable and an appropriate printing becomes such a low print speed that printing can be stably and appropriately performed on areas out of the circuits constituting the board 102 which correspond to most minute opening areas of the mask.
At Step #902, the loader 117 carries the board 102 into the stage 113.
At Step #903, the visual recognition camera 107 recognizes the position of the recognition mark of the board 102, performs a calculation of positional correction amount based on the position of the recognition mark of the screen metal mask 101, and performs the correction for the positioning of the board 102.
At Step #904, the squeegee 123 of the print head 803 moves and descends to the print start position, prints from the print start position to the print end position at the same print speed V entered at Step #901, and ascends.
At Step #905, the stage 113 on which the board 102 is placed descends so that the board 102 is transferred to the unloader 118, and the unloader 118 carries the board 102 out of the screen printing apparatus main unit 119.
However, in this conventional construction, since the speed of the squeegee 123 is constant, such a low print speed that printing can be appropriately performed on areas among the circuits constituting the board 102 which correspond to most minute opening areas of the mask is employed as the speed that allows the whole circuits of the board 102 to be printed stably and appropriately.
The reason of this is that, in the printing using the squeegee 123, if the minute opening area 121 of the screen metal mask 101 as shown in FIG. 20 was printed at a high print speed suitable for the large opening area 120 of the screen metal mask 101, as shown in FIG. 21A, the speed V of the squeegee 123 would be so fast that solder paste 122 could not fill the interior of a print hole 124 of the screen metal mask 101 positioned onto an electrode 125 of the board 102 and that, as a result, the quantity of the solder paste 122 to be printed on the electrode 125 of the board 102 would lack after the removal of the screen metal mask 101, as shown in FIG. 21B.
Accordingly, for the conventional printing apparatus, when the large opening area 120 and the minute opening area 121 are mixedly present, there is an issue that a longer process time for printing would be required because the low print speed for the minute opening area 121 is used even for the large opening area 120 that could be printed at the high print speed.
Another conventional example is explained with reference to FIGS. 12 to 14 and 23 showing a screen printing apparatus, as well as to FIG. 24 showing the flow chart of a conventional printing method.
The stage 2, on which a print-target article such as a printed circuit board 1 carried in from the preceding step is placed and positionally restricted, ascends up to the bottom surface of the stencil 4. After the printing, the stage 2 descends so that the printed circuit board 1 is carried out to the subsequent step. The leftward-printing squeegee 5a and the rightward-printing squeegee 5b are moved up and down by a squeegee ascent/descent driving means, while the squeegees 5a, 5b are also moved rightward and leftward by a horizontal mover 8c having a nut screwed to a screw shaft 8b with the screw shaft 8b being rotated as the motor 8a of the horizontal reciprocation driving means 8 rotates. As a result, the solder paste 9, which is an example of the print paste, placed on the stencil 4 is printed onto the printed circuit board 1 via the stencil 4. FIGS. 12 and 13 each show a state during a rightward printing process, and FIG. 14 shows a leftward-printing standby state. Reference numeral 812 in FIG. 23 denotes an NC unit that issues a command for driving the motor 8a, and 811 denotes a controller for controlling the whole printing apparatus.
Next, the operation of the conventional example is explained with reference to the flow chart of FIG. 24 as well as FIGS. 12 to 14 and 23.
First, a preset print speed V (mm/sec) is entered from the controller 811 to the NC unit 812 as the moving speed of the squeegee. This allows the NC unit 812 to control the motor 8a so that the left and right squeegees 5a, 5b move at the input print speed V.
At Step #101, a printed circuit board carry-in step, of FIG. 24, the printed circuit board 1 carried in from the preceding step is placed and positionally restricted on the stage 2, in which state the stage 2 ascends so that the top surface of the printed circuit board 1 placed thereon is put into contact with the bottom surface of the stencil 4.
At Step #102, a printing step, as described above, the motor 8a rotates under the control of the NC unit 812, so that the left squeegee 5a or the right squeegee 5b moves at the print speed V, by which the solder paste 9 placed on the stencil 4 is passed through the stencil 4 so as to be printed on the printed circuit board 1. In this process, it is an optimum state that the solder paste 9, during the printing process, be pressed by the squeegee 5a so as to roll and move on the stencil 4 (i.e., that the solder paste 9 be in a rolling state). Such a state of the solder paste 9 results in a successful printed pattern as well as a good screen-pass performance.
At Step #103, a printed circuit board carry-out step, upon completion of the printing, the motor 8a halts, the left squeegee 5a ascends, the stage 2 lowers, and the printed circuit board 1 is carried out to the subsequent step.
At Step #104, a decision step, it is decided whether or not the planned number of works has been completed. If it has not, the program returns to Step #101, and if it has been completed, the program comes to an end.
However, with this conventional constitution, there may occur print standby time durations due to the circumstances of the preceding and subsequent steps in which the printed circuit board 1 is carried in or out, or to rest time and the like. Then, a prolonged print standby time would cause the thixotropy ratio of the solder paste 9 to be lowered and the solder paste 9 placed on the stencil 4 to be dried such that its viscosity would increase. In this state of the solder paste 9, with the print standby canceled, when the printed circuit board 1 is carried in and positioned so that the printing is started, the left squeegee 5a moves at the print speed V previously entered to the NC unit 812. In this state, the solder paste 9, whose thixotropy ratio has decreased and whose viscosity also has increased, would be printed in a state other than appropriate rolling. This would cause the occurrence of such faults as solder paste chipping, solder shortages, or blurs, posing an issue that defective boards may be produced.